A polymer hydrogel contains water as a main component, so that it is highly safe and it can be inexpensively produced. The polymer hydrogel places a low burden on the environment, so that it is attracting attention as a material for a soft material and it is utilized for an aromatic, a jelly, a paper diaper, and the like.
However, many types of polymer hydrogels have a heterogeneous structure in which the crosslinkage density has a distribution, and they are mechanically brittle.
To compensate for such a drawback on the mechanical properties, a polymer/nano fine particle composite-type hydrogel has been attracting attention.
As a polymer/nano fine particle composite hydrogel having excellent mechanical properties, there is disclosed a nano composite gel obtained by radically polymerizing an acrylamide-based monomer in the presence of delaminated clay particles in water (Patent Document 1 and Non-patent Document 1). As an example of a similar disclosure, there is also known a nano composite gel composed of a polymer containing a group having a carboxylic acid salt structure or a carboxylic acid anion structure in some part of the poly(meth)acrylamide and clay particles (Patent Document 2). Furthermore, there is disclosed a hydrogel obtained by mixing sodium polyacrylate, clay particles, and a polyion dendrimer having a cationic functional group at a terminal thereof (Patent Document 3 and Non-patent Document 2).
On the other hand, a dry clay film composed of a polyacrylic acid salt that is a commonly-used polymer and clay is known and is studied as a surface protecting material (Patent Document 4).
A study on a viscosity change of an aqueous dispersion of sodium polyacrylate and clay is known (Non-patent Document 3). This is not a study on a hydrogel having excellent mechanical properties.